1. Field of the Invention
The present invention relates generally to a method of clean removal of ions of mass m+1 from a Paul ion trap mass spectrometer.
2. Description of the Prior Art
Before performing certain investigative procedures on ions having a desired mass m in an ion cage, ions of a desired mass m must be at least partially "isolated" by removing undesired ions of a mass other than mass m from the ion cage, such that only ions of mass m remain in the ion cage. Such procedures include, for example, generating daughter-ion spectra in the ion cage and investigating ion/molecule reactions of specific ions.
Ions must be initially generated from gaseous starting substances in order to then isolate ions of the desired mass m. The ions are usually generated inside the ion cage. Typically, the ions are generated by electron impact ionization, in which a beam of electrons is directed into the cage. Other methods of ionization can also be used, such as photon ionization using lasers or chemical ionization.
In all of the known ionization processes wherein ionization occurs inside the ion cage, ions having a mass other than the desired mass m are generated simultaneously with ions of mass m, even when pure gaseous starting substances are introduced into the ion cage. However, ions of the desired mass m are necessary in order to perform the above-mentioned investigative procedures. Further, where complex mixtures are used as starting substances, such as in the investigation of pyrolysis products, isolating ions of the desired mass m is even more difficult than isolating ions generated from a substantially pure starting substance.
Of the known ion isolation methods, the oldest method of eliminating all undesired ions uses a corner of the ion stability graph. If the electrodes of the ion cage are supplied with a precisely adjusted DC voltage and high-frequency (HF) amplitudes, the working point for the ions to be isolated can be localized to a corner of a known a/q stability graph of a quadrupole cage. Therefore, all ions other than ions of the desired mass m, or undesired ions, will be outside the stability region. Kinetic energy is transferred from the high-frequency (HF) field to the undesired ions, causing undesired ions to leave the quadrupole cage.
The quadrupole cage can also be operated in the above-described mode during ionization. However, operating the quadrupole cage in the above-described mode during ionization results in a low yield of ions of the defined mass m, since high losses of ions occur in this region. Further, the method cannot be applied to non-linear quistors with octupolar fields. Non-linear quistors have certain advantages over other kinds of quadrupolar cages, as the non-linear resonance conditions run through the corners of the stability region.
In a non-linear quistor, a non-linear field pattern from the center of the quistor to the annular electrode and the end cap electrodes is generated by superposition of higher-order multipolar fields. For example, given a quistor, a weak octupolar field is superimposed on a quadrupolar field. In this case, ion resonance occurs if the secular frequencies f.sub.r and f.sub.z of the ions in the r and z direction satisfy the equation f.sub.r +f.sub.z =Fs/2, where Fs is the frequency of the storage high-frequency (HF). The resonance condition is normally written as .beta..sub.r +.beta..sub.z =1, which defines a curve in the a/q stability graph that extends through both corners used for ion isolation and intersects the line a.sub.z =0 at approximately q.sub.z =0.78.
The non-linear resonance has virtually no effect for ions undergoing extremely weak oscillation or for ions remaining in the resonance state for a short time. However, if the amplitude of oscillation of the secular motion increases or if the ions remain in the resonance state for a longer time, kinetic energy from the storage HF is transferred to the ions. The effect is greater the closer the working point is to the edge of the stability region. The amplitude increases exponentially and the ions leave the cage, mainly by striking the electrodes. Further, the resonance condition satisfied by the equation .beta..sub.r +.beta..sub.z =1 intersects the two useful corners of the stability graph, resulting in an almost complete loss of the desired ions.
Another known method for isolating ions, in which HF voltages are used exclusively (i.e., no DC voltage), is discussed in U.S. Pat. No. 4,749,860. An HF ejection voltage is applied at a fixed frequency between the end caps of the ion cage. The frequency is selected such that ions having a mass one unit higher than the desired mass m, or ions of the mass m+1, are ejected. Ejection of the m+1 ions is achieved if the secular frequency is in resonance with the ejection frequency. The amplitude of the HF voltage is then increased, so that all ions of lower mass are eliminated when they cross the instability boundary .beta..sub.z =1. The process is continued until the mass m-1 has been eliminated.
Using the same method, when the HF amplitude is increased, the fixed ejection frequency ejects ions of progressively higher masses, beginning with the mass m.sub.1, as the secular frequencies of these ions is altered with the HF amplitude. Ions having progressively higher masses experience resonance and are ejected. The accuracy of this ion ejection process, however, is limited. For example, if the neighboring mass m+1 is to be completely eliminated within a reasonable time, the losses of mass m will be high (more than 90%). On the other hand, if the mass m must be obtained with a high yield, the mass m+1 will not be completely ejected. The process is also disturbed considerably by space charge effects when a number of ions are contained in the cage.
Yet another known method for isolating ions was proposed by R. Yost et al. during the AMS meeting in 1991. In this method, both instability limits .beta..sub.z =1 and .beta..sub.z =0 are used by applying suitable HF amplitudes followed by positive or negative DC voltages. This method is superior to the two previously discussed methods for isolating ions. However, the instability limit .beta..sub.z =0 is not sharply delimited, but rather forms a very soft transition. Thus, a small percentage of the masses m+1 and m+2 are still present, even if the proportion of these ions in the starting substance is relatively low.
In yet another method for isolating ions, a special form of sextupole and/or octupole potential is superposed on the quadrupole potential by providing electrodes having a special shape. The shape of the electrodes increases the withdrawal rate of analyzed ions without altering the mass resolution capacity. This method is discussed, for example, in German Patent Application P 40 17 264.3-33, which is not a prior publication.